The present invention relates to combined cycle electric power plants and more particularly to improved gas turbine temperature limit controls especially useful in the operation of gas turbines in combined cycle electric power plants.
In the generation of electric power with the use of gas turbines as prime movers or suppliers of heat energy, it is desirable to provide automatic startup and loading control with manual backup control. An outlet temperature control loop is one of the loops normally included in an automatic gas turbine control to prevent the gas turbine from operating at excessive temperatures which would otherwise cause excessive metal damage. Such a loop typically controls the turbine outlet temperature to the temperature limit value as a setpoint once the loading on the turbine causes the outlet temperature limit value to be reached under the existing ambient conditions. Although the same temperature control loop or a similar one could be employed in manual backup control, it is often desirable for reasons of economy and operating flexibility to provide a reduced level of automation for the backup mode outlet temperature control without losing the basic turbine protection intended to be provided. The outlet temperature control may be based on exhaust path temperature signals or blade path temperature signals or some combination of the two. The term outlet temperature is herein intended to include blade path temperature and exhaust temperature.
In combined cycle plants, it is especially desirable that the outlet temperature control be provided during automatic and manual control in such a way that the rate and amplitude of outlet temperature cycling be restricted to avoid excessive and unnecessary inlet gas temperature cycles on the heat recovery steam generator apparatus and accordingly to avoid excessive and unnecessary steam temperature (or condition) cycles at the inlet to the steam turbine apparatus. Further it is desirable expecially in combined cycle plants that temperature limit action be provided in such a way that manual turbine load control is made possible. Therefore, in providing manual gas turbine backup control for gas turbines in combined cycle plants which are normally attended by an operator, it is desirable that the gas turbine outlet temperature be efficiently and reliably limited with a relatively reduced level of automation, consistently with a provision for manual load control and avoidance of excessive rates and magnitudes of outlet temperature cycling about the limit outlet temperature value. In stand-alone gas turbine electric power plants which are often unattended, a similar kind of manual mode outlet temperature backup control can be desirable.
In the prior art, the previously referenced patent application Ser. No. 319,114 discloses an automatic digital computer control system for gas turbines in which some degree of automatic load control is provided and in which setpoint exhaust and blade path temperature limit control is provided with a temperature anticipation function which in effect limits the rate at which fuel flow increases are made as the exhaust or blade path temperature limit is approached. Manual backup control is not disclosed in that application, and accordingly the relationship of outlet temperature limit control to load control in automatic and manual modes of operation is not addressed therein. A manual backup control usable with a digital computer control for gas turbines like the one disclosed in Ser. No. 319,114 is disclosed in another copending patent application Ser. No. 476,182 entitled "Local Maintenance Controller For Gas Turbine Power Plants Having A Primary Control System" filed by R. A. Yannone et al on June 4, 1974, and assigned to the present assignee. In that patent application, outlet temperature limit control in the manual backup controller is provided with integrator setpoint control and low select action.
Among various other prior art schemes for placing limit control on gas turbine outlet temperature in electric power plants, a control system which includes both electronic and pneumatic subsystems is disclosed in a copending patent application Ser. No. 371,621 entitled "Gas Turbine Power Plant Control Apparatus Including Automatic Load Pickup", filed by R. A. Yannone and J. J. Shields on June 20, 1973, and assigned to the present assignee. In that electropneumatic control sustem, outlet temperature limit is controlled by a temperature setpoint control in the backup and primary control modes. In early gas turbine control systems, manual speed/load controls have been provided with temperature limit control but such systems were relatively simple and had little or no automation.
The description of prior art herein is made on good faith and no representation is made that any prior art considered is the best pertaining prior art not that the interpretation placed on it is unrebuttable.